Got it – we’re still waiting to get our hands on a Zwift Hub trainer at our HQ in Reno, but one of my colleagues does have one that we have done a bit of testing on. We’ll have more info on this trainer once we have one at the TR office. 
When you go to calibrate the trainer, could you try pedaling through the target speed? My colleague reported that reaching the target speed and stopping immediately may result in a “calibration failed” message.
In other words, try maintaining the target speed or above for about 20 seconds as you mentioned in your own post.
Alternatively, if you aren’t able to get things working through the TR app, you can also use the Zwift Companion app to calibrate your Zwift Hub. The calibration data gets stored locally in your trainer, so you’ll still be good to use it with TR once the calibration is complete!
Feel free to let me know if you have any other questions on this topic, and we’ll post updates on the calibration process for the Zwift Hub here as we get them. 